The overall objective of this project is to develop an understanding of the physical - chemical relationship between polymorphism and bioavailability. Steroids were chosen as the initial drugs for this study because they are clinically important drugs which exist in several polymorphs and exhibit bioavailability problems. The knowledge gained from this study will resolve the question of whether polymorphs are responsible for the bioavailability problems of steroids. In addition, experiments are planned which will use the physical - chemical bonding exhibited by the crystal packing of molecules in polymorphs as a guide to the design of new dosage forms containing drug-polymer mixtures.